1. Field of the Invention
The present invention relates to a process for manufacturing a continuous thread formed of a large number of continuous filaments of which at least some are covered with a resin mixture in the liquid state which reacts or cross-links so as to harden when subjected to ultraviolet radiation. More precisely, the invention is directed to a process for manufacturing a continuous thread which is at least partially formed of filaments intended to serve as a reinforcement, such as glass filaments.
2. Description of the Related Art
It is known to impregnate a glass fiber based fabric or roving with a resin mixture which reacts when exposed to ultraviolet radiation. This is the case in the manufacturing process described in French patent application FR-A-2 336 776 and its certificate of addition FR-A-2 382 079.
In accordance with these documents, a glass fiber roving, extracted from a winding, passes through a bath of reactive mixture before running parallel with a tube emitting ultraviolet radiation. In order for the resin mixture impregnating the roving to polymerize and crosslink, the velocity at which the roving moves in front of the emitter tube is relatively low.
This type of indirect process is only practical when the low velocity of less than one meter per second can be considered less disadvantageous than the great advantages it offers.
This is unlike a direct process in which the mixture is deposited on the thread during the manufacturing process of the glass thread itself, and in which the thread velocity, of more than several meters per second, is imposed by the process.
It is this type of process which is used, for example, for obtaining optical fibers. Each optical fiber is obtained by the mechanical drawing of the end of a preform softened by heat. As soon as it is formed, the optical fiber must be protected from moisture and any contact likely to cause the appearance of defects on its surface. For this reason, a reactive mixture is applied to the fiber such that it is completely covered. Immediately after this coating has been deposited, the fiber is exposed to ultraviolet radiation emitted by one or a plurality of tubes disposed parallel to its path, before it is wound. Since the drawing velocity of an optical fiber can reach 5 to 10 meters per second and the layer of mixture which is to be deposited is not inconsiderable, the polymerization speed of the applied mixture must be high. A mixture of this type comprises expensive constituents and its use sometimes requires the use of safety measures. A process of this type is described for example in U.S. Pat. No. 4,099,837.
Within the field of glass fibers intended for the reinforcement of resinous materials, it is likewise known to deposit a mixture which reacts under the effect of ultraviolet radiation on the surface of the filaments during the fiber-drawing operation. The filaments obtained by the mechanical drawing of glass streams flowing from openings in a die plate are coated with the reactive mixture before being combined to form a thread which is exposed to ultraviolet radiation over part of its path before being wound. A process of this type is described in EP-B1 243 275.
This process is advantageous, as are all direct processes, insofar as it enables a product to be obtained which avoids the need for a supplementary step which is expensive in terms of time, material and space. However, it is subject to certain constraints.
The drawing velocity of the filaments, which is generally far higher than that of optical fibers, requires extremely reactive mixtures to be used.
Furthermore, the ultraviolet radiation source or sources used is/are regulated such that the radiation is focused on part of the path of the thread in a narrow area. The polymerization and/or cross-linking of the mixture covering the thread is uniform if the thread does not deviate from this area. At high velocities, however, the thread can vibrate, which will move it slightly away from this area, which modifies the reaction rate of the mixture over part of the thread.
In addition, the vibration of the thread causes localized temporary separation of filaments or groups of filaments in the irradiation area. The polymerization and/or cross-linking, which is at least partial, occurring during this separation phase does not produce a thread of which the integrity is constant over its entire length.